We will investigate the mechanism of virus-induced cell fusion by several methods. We will do a comparative study of the cell-fusing activities of several types of RNA-containing, enveloped viruses, including a paramyxovirus, a murine leukemia virus, a corona virus, and a myxo-like virus (respiratory syncytial virus). Using biochemical fractionation techniques and high resolution electron microscopy, we will attempt to isolate and characterize a component with cell-fusing activity from virions or from virus-infected cells. A new method for detecting biological activities of subviral components will be developed. We will study the interaction of virions with the plasma membranes of cells sensitive to virus-induced cell fusion. The hemolytic reaction of a paramyxovirus will be studied as a model for the initial step in the sequence of reactions leading to cell fusion. We will propose a new theory for a mechanism of viral oncogenesis based upon the newly discovered cell-fusing activities of some oncogenic viruses.